Ovens for dielectric heating



y 1953 J. F. NAYLOR ETAL 2,842,650

OVENS FOR DIELECTRIC HEATING Filed Aug. 17, 1955 FlC-3. 2:-

Joseph Francis Naylor Raymond Northmore Christopher Evan Mundall Tibbl Invenforg Aifomtg Tnited States Patent Ofitice OVENS FOR DIELECTRIC HEATING Joseph Francis Naylor, Earlestown, Newton-le-Willows, Raymond Northmore, Lydiate, near Liverpool, and Christopher E. M. Tibbs, Wokingham, England, assignors to West Lancashire Radio& Television Lab oratories Limited, Lydiate, near Liverpool, T. &. T. Vicars Limited, Earlestown, Newton-le-Willo-ws, and Radio Heaters Limited, Wokingham, England, all British companies Application August 17, 1955, Serial No. 528,912

Claims priority, application Great Britain September 6, 1954 Claims. (Cl. 21910.69)

This invention relates to ovens for high-frequency dielectric heating wherein the articles or pieces to be heated are continuously passed through the oven on a conveyor band. Such ovens are used for baking confectionery, biscuits, cakes, rolls and loaves but may also be employed for baking various other articles such as foundry cores, ceramic articles of all kinds and also for drying materials such as cheeses of rayon, cotton or wool, textile fabrics, paper, cardboard, powdered chemical substances and so forth.

Such ovens are commonly constructed almost entirely of steel and often have continuous moving-band conveyors, of sheet steel or of a woven steel wire mesh, which are conducted over skid plates arranged to carry the steel band or mesh web in perfect alignment or flat condition. Frequently the conveyor is arranged so that the whole of its track within the oven shell can be raised and lowered. In addition, these ovens are sometimes provided with internal steel ducts for hot air and feeding arrangements for the hot air. For ovens of this kind, a diflicult problem arises when it is attempted to design the apparatus with a low loss return path for the radio frequency space current flowing between the electrodes and the side walls and top of the oven. It will be appreciated that radio frequency currents flowing through steelwork generate a good deal of heat which represents a very serious loss of power. In actual fact, when relatively large electrodes are employed, the space currents in the ovens may be extremely heavy and the Whole output of the radio frequency generator provided may easily be taken up in heating up the steel framework of the unit itself.

The object of the present invention is to reduce the loss of energy in the form of waste heat to the lowest possible minimum and according to the invention, this is achieved by mounting the pair of balanced electrodes within a metal screening hood so that radio frequency space currents other than those flowing through the conveyor band tend to flow to the screening hood.

The radio frequency space current which flows from one of the electrodes through the articles being heated to the conveyor band and passes along the latter before returning to the other electrode, gives rise to losses in the conveyor band which are quite appreciable. These losses are inevitable but may be reduced somewhat in certain cases by providing a non-magnetic stainless steel plate below the conveyor band.

In order that the invention may be better understood, a particular form thereof will now be described by way of example only, with reference to the accompanying drawings.

In the drawings:

Figure 1 is a perspective view showing the arrangement In Figuresl and 2v a conveyorband 3 passes through,

anoven 4-over anon-magnetic sheet 5 of stainless steel which rests on channel members-6. The latter are themselves supported on brackets 7 which are fixed to the walls of the-oven. The returning portion of the conveyorband is-shown 'atthe bottomof Figure 2.

Abovethe conveyor band-3 balanced electrodes Sand 9 are suspended by means of insulators 10 from a screening hood 11. The screening hood 11 is itself suspended by means of four spindles 12 from the roof ofthe oven. The spindles 12 pass into sleeves 13 and are provided with racks which co-operate with pinions 14,- these latter extending through slots in the sleeves 13. The two pinions 14 shown in Figure 2 are mounted on a shaft 15 which extends outside the oven and on which is mounted a hand wheel 16. The remaining two pinions are driven from the shaft 15. By rotating the hand wheel the screening hood 11 and the electrodes 8 and 9 can be moved bodily up and down, the lower position of the electrodes being shown in chain-dotted lines in Figure 2. By moving the electrodes towards or away from the conveyor band in this manner, the power output of the apparatus can be controlled.

The electrodes 8 and 9 are connected to a radio frequency generator 17 by means of strip conductors 18 and 19. The screening hood is cut away at 20 to ensure that the conductors 18 and 19 do not foul the screening hood when the latter is moved to its lower position.

In the embodiment shown, the spindles 12 are mounted on the screening hood 11 by means of panels 21 of silicon glass, and the screening hood is in this way insulated from the rest of the oven.

As an alternative to the construction described, the electrodes may be mounted on insulators in a fixed hood and the conveyor band beneath the hood may itself be moved up and down.

The hood is made of material having good electrical conducting properties such as sheet aluminum, copper or brass and may be formed of a single sheet of metal or built up from a number of sheets brazed or soldered together.

It will be appreciated that the invention enables currents flowing through the framework of the oven to be largely avoided. If the hood is insulated electrically from the rest of the oven, it also serves as an internal screen and greatly reduces the chances of serious interference radiation from the equipment. Furthermore the provision of balanced electrodes removes the need to cause return current to flow back from the conveyor band through an earthed plate to the radio frequency generator.

It will be appreciated that changes in the details, which have been herein described and illustrated for the purpose of explaining the nature of the invention, may be made by those skilled in the art without departing from the principle and scope of the invention as determined by the appended claims.

We claim:

1. An oven for the radio frequency di-electric heating of articles comprising an oven casing, a conveyor band passing through said oven casing for carrying the articles to be heated, a pair of balanced electrodes mounted above said conveyor band within said casing and adapted for connection to a radio frequency generator to efllect dielectric heating of articles passing below the electrodes on said conveyor band, and a screening hood of a material having good electrical conducting properties enclosing said electrodes at the top and sides of the latter itatented July-8, 1958v so that radio frequency space currents other than those flowing the conveyor band tend to flow to said screening hood, thereby to avoid the dissipation of the space currents in said oven casing.

2. An oven as in claim 1; further comprising means electrically insulating said screening hood from the rest of the oven.

3. An oven as in claim 1; further comprising means suspending said electrodes from said screening hood, and means operative to effect relative displacement of said hood and electrodes and of said conveyor band.

4. An oven as in claim 3; wherein said means operative to effect relative displacement of said hood and electrodes and of said conveyor band includes spindles extending from said hood and having racks formed therei on, pinions meshing with said racks to effect displacement of the latter in response to rotation of the pinions, and means actuable from outside of said oven casing for effecting rotation of said pinions.

S. An oven as in claim 4; further comprising silicon glass panels interposed between said spindles and said hood to electrically insulate the latter from the rest of the oven.

References Cited in the file of this patent UNITED STATES PATENTS V 

